Tube-type electrostatic precipitator

ABSTRACT

A tube-type electrostatic precipitator in which the interior of a tube is provided with a film of conductive liquid to which the particles are attracted and which surrounds a corona discharge electrode. The outside of the tube is provided with fiberglassreinforced synthetic-resin bands while the interior of the tube is sandblasted or otherwise roughened to a minimum roughness of 15 microns.

Matted States Patent 1 Hardt et a1.

TUBE-TYPE ELECTROSTATIC PRECIPITATOR Inventors: Lothar Hardt, Eschborn; Heinz Miiiler-Wartenberg; Hans Rausehenberger, both of Frankfurt am Main, all of Germany Metallgesellsehafit Aktiengesellschatt, Frankfurt/Main, Germany Filed: Jan. 3, 1972 Appl. No.: 214,740

Assignee:

Foreign Application Priority Data July 10, 1971 Germany P 21 34 576.6

11.8. C1 55/146, 55/148, 55/152, 55/155, 55/DIG. 38,161/93, 161/164 Int. Cl. B036 3/49 Field of Search 55/146, 148, 150, 55/151,152, 154,155, 156, 157, DIG. 38; 161/93, 164

[56] References Cited UNITED STATES PATENTS 1,371,995 3/1921 Nesbit SS/DIG. 38

1,440,887 l/l923 Nesbit 55/127 X 3,248,857 5/1966 Weindel et a1 55/155 X FOREIGN PATENTS OR APPLICATIONS 538,860 6/1955 Belgium 55/155 883,876 12/1961 Great Britain 55/151 Primary Examiner-Dennis E. Talbert, .Ir. Atz0rneyKarl F. Ross [57] ABSTRACT A tube-type electrostatic precipitator in which the interior of a tube is provided with a film of conductive liquidto which the particles are attracted and which surrounds a corona discharge electrode. The outside of the tube is provided with fiberglass-reinforced synthetic-resin bands while the interior of the tube is sandblasted or otherwise roughened to a minimum roughness of 15 microns.

10 Claims, 4 Drawing Figures 4 lam /tmminvm 3.765.154

' sum 1 or 2 FIG. I

PATENTED URI I 6|973 SHEET 2 [IF 2 FIG. 2

TUBE-TYPE ELECTROSTATIC PRECIPITATOR FIELD OF THE INVENTION BACKGROUND OF THE INVENTION In the art of removing particles from a gas stream,

electrostatic methods have gained increased prominence in recent years. Electrostatic precipitators, dust or particle removers, filters and the like, may be of the dry or wet type. In a system of the dry type, the gas passes through a compartment flanked by collector electrodes and having an array of corona-discharge electrodes substantially centered in the compartment. The solid particles contained in the gas stream pick up electric charge by ionization and are attracted to collector electrodes of opposite polarity. The collector electrodes are periodically rapped to shed the accumulated dust which may fall into troughs or bins for removal.

in the wet process, the collector electrode" is formed by a film of liquid moving along a surface and to which the particles are attracted. It has been suggested in a system of the latter type to form the flow compartment as tubes whose inner walls serve to guide the film of liquid and surround coaxially a corona-discharge electrode.

Among the problems encountered with systems of the latter type, have been the extreme corrosity of many of the gases which are to be processed and may derive from chemical plants, the inability to operate at elevated pressures and the unsatisfactory electrical characteristics. When lead, titanium and stainless steel are used as the material for the tubes, they are highly expensive and not sufficiently unreactive in many instances to be desirable.

We should acknowledge that it has been proposed to provide synthetic-resin tubes, eg of polyvinylchloride, polyesters or polytetrafluoroethylene as the tubes of a wet electrofilter, but such use of synthetic resin has diadvantages as well. Thus, when it was desired to make the inner surface or wall conductive, a coating of metal or graphite has been applied. This'coating readily wears off and becomes ineffective. Efforts to increase the conductivity at the surface of interest have proved to be unavailing. When metal powder, for example, was incorporatedinto the synthetic resin of the tube. In most instances, a satisfactory conductivity could not be obtained without increasing the proportion of metal powder to the point that the disadvantages of metallic tubes were reinstated. Also, lesser proportions of metal did not yield sufficient conductivity.

As a consequence, considerable effort was expended in designing wet electrofilter tubes whose inner walls were rendered conductive by the application of films of a conductive liquid thereto. The conductivity of the liquid, of course, can be obtained by the dissolution of salts therein and/or by the absorption by the film of components, such as sulfur trioxide which interact with water or other film-forming liquids to produce an ionic solution. These liquid-film electrofilters also had some significant disadvantages. Firstly, it was not possible with conventional systems to maintain a completely continuous film over the entire surface area of the inner wall of the tube. In many instances, the surface of the tube was exposed and the liquid film broken up into streamlets or bands. In the areas in which the liquid film recedes from the exposed surface, discharges may be generated which may damage the synthetic resin tube in those areas from which the liquid has disappeared. It is, therefore, important for efficient operation of a liquid-film electrofilter to maintain the conductivity of the film and this has not been posible with conventional synthetic-resin tubes. Another disadvantage which has arisen in the use of synthetic-resin tubes in electrofilters of the character described, is that the synthetic resins have only a limited useful temperature range. As a consequence, the filter can be used only within this range and is limited in applicability by the properties of the synthetic resin employed. Polyvinylchloride, for example, admits of an upper temperature limit of C for the electrofilter. Thus, if the gas to be treated is at a higher temperature, preliminary cooling must be carried out. A direct cooling is generally provided in which the water or some other solution is sprayed into the system and carries away part of the heat of the gas in a direct gas/liquid heat transfer. This increases the amount of liquid which must be processed although it improves the continuity of the film.

In reviewing the state of the art from which the instant improvement has developed, much should be made of the use of synthetic resin elements, e.g. of modified or nonrigid polyvinylchloride, as inserts between the supports and the collector elements of electrofilters. Even these systems have disadvantages, primarily as a result of damage to the yieldable supports.-

OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved liquid-film electrostatic precipitator wherein the aforementioned disadvantages can be avoided.

Another object of the invention is to provide a liquidfilm electrostatic precipitator of the tube type having improved liquid-film distribution uniformity and dust or smoke pickup.

Still another object of the invention is the provision of a tube/electrode structure in a liquid-film electrofilter or electrostatic precipitator in which temperature and pressure stresses are more readily compensated or absorbed.

SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, in an electrostatic precipitator of the wet or liquid-film type in which the dust-collector tubes are constituted of a synthetic resin and are externally reinforced with fiberglass-containing hardenable syntheticresin bands preferably wound helically around the tube. In addition, the interior of each of the tubes is artificially roughened, i.e., treated by sandblasting or some other material-removal or forming technique to impart a roughness to the interior surface encountered by the liquid of a minimum of 15 microns. The term roughness" is used herein to designate the actual depth of craters, grooves and channels formed randomly by sandblasting or the like as measured from the high point adjoining each such recess, A minimum roughness" is thus the measurement of the smallest depth encountered in a random survey of the surface by depth-gauge techniques. The value is obtained in microns. lt should be observed that above the minimum roughness value, the roughness distribution may assume normal statistical values with a large proportion of the recesses lying at a distance below the high point of, say, the mean value of several times the minimum.

Some recesses may be somewhat deeper. For the purposes of the present invention, however, only the minimum roughness has been found to be critical.

Surprisingly, neither the use of glass-fiber-reinforced synthetic-resin band along the exterior of the electrofilter tube, nor the roughness augmentation mentioned earlier have along provided the improved results obtained when both expedients are practiced together. While we do not wish to be bound by any theory in this regard, it is altogether possible that a synthetic-resin electrode filter tube acts as a membrane under prior-art conditions and is deformed, distended or distorted in use to break down the interior film of liquid. With the system of the present invention, however, continuity of the liquid film is maintained in a wholly unexpected and effective manner.

The glass-fiber reinforced bands around the synthetic-resin tube thus appear to increase its stability and its compressible strength, i.e. its resistance to deformation under pressure, and allows the entire electrofilter housing as well as the collecting tubes to be of increased height. The use of fiberglass reinforeced tube, moreover, provides a substantial saving in weight by comparison to steel or lead electrostatic precipitators.

In more general terms, the present invention provides an electrostatic precipitator having an upper portion, an intermediate portion containing the collecting tubes, and a lower portion provided with a gascollection manifold communicating with the tubes. Means is advantageously provided to allow relative axial displacement of the tube and parts of the housing or its support structure and, in accordance with conventional practice, a corona-discharge electrode passes generally centrally through the interior of the tube.

We have already mentioned the surprising improvement in the continuity of the liquid film which is obtained when the interior surface of the tube is sandblasted to an artificially induced roughness of microns. It should be noted that it is important to roughen the surface of the tube in this manner in spite of the fact that the interior of the tube may naturally have various projections and recesses. For reasons which we do not fully understand, the continuity of the liquid film depends in large measure upon the sandblasting type of roughening which is produced when small particles are projected at high velocity against the internal tube surface. The roughening step eliminates the liquidshedding character of the surface and allows a thin but coherent liquid film to form over the entire surface of the collecting tube. The resistance of such continuous films to breakdown is increased when, in accordance with the present invention, the gas to be purified is fed through the tubes downwardly, i.e., from the upper portion of the housing to the lower portion through the matrix of collecting tubes.

Each tube thus consists of a synthetic-resin sleeve around which the fiberglass-reinforced synthetic-resin band is wound and to which the reinforcing band is bonded, the inner surface of the sleeve being roughened as described earlier. The use, directly, of glassfiber-reinforced tubes is not practical because the roughening step tends to expose the glass fibers and cause liquid to be drawn by the capillarity of the glass fibers into the wall of the tubing. The liquid crystallizes within the network of glass fibers and may destroy the tube by expansion therewithin. A further advantage of the present invention resides in the fact that the electrostatic precipitator of the present invention can be easily assembled and disassembled as may be required. To this end, the upper portion and the intermediate portion are coupled together via flanges and a similar set of flanges is provided between the lower portion and the intermediate portion.

Within the housing, we provide supporting structures extending generally horizontally or transversely to the gas flow direction, the support means being in the form of traverse members, support plates or brackets, have walls or partitions subdividing the interior of the housing into the several gas spaces. Advantageously, these supporting portions of the wet electrofilter are provided within the housing from a metal core coated with synthetic resin. This construction provides high stability and strength for the supporting members.

According to still another feature of the invention, the lower portion of the precipitator is formed with an upwardly convex or rough-shaped condensate bottom at which the liquid is conducted outwardly. The condensate body is provided with a multiplicity of upstanding tube stubs which are received coaxially within the collecting tubes. The rough-like configuration of the condensate body has the advantage that precipitated dust, liquid and sludge can pass from the tubes which terminate above the upper ends of the tubular stubs, outwardly to the collecting vessels. The gas, however, can pass without difficulty through the partition via the stubs. It has been found to be desirable to space the inner face of the collecting tubes from the outer faces of the coaxial tubular stubs apart by at least 5 mm and to provide the stubs with axially extending radially projecting ribs angularly equispaced about the stubs and engaging the inner face of the wall. This assures centering of each tube upon the respective tube stub. Best results have been obtained with three such ribs. It will be apparent that the individual collection tubes may simply be thrust over the upstanding stubs and are centered thereby, while being spaced from one another. The gap between each tube and the stub serves to discharge the liquid and the solids entrained thereby and maintains an equipotential electrical characteristic with respect to the condensate bottom and the tubes. As a result, the liquid is permitted to drop from the lower edge of each collector tube without electrical disruption or dispersion and without creating a sparkdischarge condition.

Still another feature of the present invention resides in sealingly mounting each of the collector tubes in the upper portion of the precipitator housing in a selfsealing construction whereby the sealing force derives from the weight of the collector tube itself. To this end, a support partition may be provided across the upper portion of the housing and formed with openings through which the individual collector tubes can be inserted from above, each of these openings may be formed with an upwardly convergent frustoconical seat against which an O-ring is clamped by a shoulder formed on the collecting tube. Here again, the sealing means serves as a spacing and aligning element. The grounding of the individual collector tubes is achieved by synthetic resin textile bands cemented to the inner face of each collector tube and cemented to or simply permitted to rest upon the condensate bottom.

Because the artificial roughening of the particlecollection tubes produces a closed or continuous liquid film and the continuity of this film is maintained during the filtering or dustremoval process, the precipitated liquid and particles or salts contained therein are able to flow via the synthetic-resin textile bands onto the condensate bottom or wall. The liquid thus provides the conductivity of the textile bands which is essential to apply the charge of the precipitating electrode surface (inner face of the tubes) to the condensate bottom. Another advantage resides in the fact that the textile bonds can be adhesively secured directly to both the tube bottom or tube sheet and the collecting tubes associated therewith. This further reduces the tendency to arcing between the tube and the tube sheet and precludes disruption of the liquid transfer to the latter.

In accordance with yet another feature of the invention, the corona-discharge electrodes are elongated elements extending centrally through the collector tubes, preferably along the axis thereof. The corona-discharge electrodes are composed of corrosion-resistant materials, such as lead, titanium, tantalum or tungsten, or have coatings of such corrosion-resistant metals upon a core of some other material. Metallic coronadischarge electrodes are preferred because of the resistance of such materials to aggressive gases which may be filtered in the apparatus. The named metals are especially effective when the treated gases contain sulfuric acid, chlorine, hydrogen chloride and the like or their precursors such as sulfur dioxide and sulfur trioxide. The corona-discharge electrodes are also provided with corona points, tongues or projections which may be concentrically arranged within the collector tube and may be axially and angularly spaced apart therein to increase the corona output. The projections also have the advantage that the gases can traverse the tubes at high velocities in view of thegreater degree of corona formation, without reducing the filtering efficiency. The synthetic-resin sleeve according to the invention is composed of polyvinyl chloride which is reinforced by the fiberglass-containing polyester bands. Surprisingly, this material is preferred over polytetrafluoroethylene which has a higher working temperature but is so expensive as to scarcely enter consideration. It is important for the purpose of the invention that gas bubbles and voids be excluded from the interface between the fiberglass-reinforced polyester bands and the the polyvinyl-chloride sleeve during the application of the bands to the latter. The elimination of such voids reduces the possibility of separation of the reinforcing bands from the inner sleeve during application of pressure to the tube under conditions which may cause distension.

The electrostatic precipitator or electrofilter accordcation of chlorine gas as is generated by fused-salt hydrolysis, for the cleaning of sulfur-dioxide roasting gases generated in sulfuric-acid plants for the cleaning of the exhaust gases produced from titanium-dioxide calcination in a rotary furnace, the exhaust gases containing hydrogen chloride and sulfuric acid produced in the synthesis of chlorosulfonic acid and in the chlorination of nonferrous metals, for the exhaust gases generated in the production of aluminum chloride and containing hydrogen chloride and aluminum hydroxide contaminants, the output gases of a contact-process plant for the production of sulfuric acid in which case sulfuric acid droplets are removed, the gases discharged from a plant for roasting vanadium-containing substances, in which case hydrogen-chloride droplets are collected, and the exhaust gases from the combustion of industrial and municipal wastes. It can be used in the cleaning of sulfuric-acid-containing waste gases, from calcining furnaces for zinc hydroxide or the sulfuric-acid waste gases from reaction vessels of different chemical processes.

DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

SPECIFIC DESCRIPTION In FIG. 1 of the drawing, we show a plastic-tube, wet electrofilter or precipitator for the purification of gas by removal of solid and/or liquid particles therefrom which comprises an inlet sheet 1 at its upper end which is provided with a flange la adapted to be connected to the duct work, pipes or apparatus of, for example, a chemical plant. The inlet 1 opens with a downwardly divergent frustoconical diffuser 2 whose flange 2a is connected by'a flange 4a to the upper member 4 of the housing. Between the flanges 2a and 4a, a perforated plate 3 is clamped to span the diffuser mouth and further distribute the gases uniformly over the full crosssection of the housing.

The cylindrical housing member forming the upper portion 4 of the apparatus is provided with a pair of diametrically opposite radial ducts 5 and 5 opening horizontally into the interior of upper portion 4. The ducts support insulator assemblies 6 and 6a provided with the electric supply conductors feeding electric current to a traverse bar 7 supporting the upper ends of the coronadischarge electrodes represented collectively at 8. Bar 7 is thus conductive. At its bottom, the upper portion 4 is provided with a shoulder 4b above a flange 40 by means of which the upper portion is connected to the intermediate housing portion 23. The shoulder 4b carries a plate 9 in the form of a tube sheet and can be described as a bottom, the tube sheet having the configuration shown in FlG. 2. having the configuration shown in FIG. 2. More specifically, the tube sheet 9 is provided with an array of through-going vertical openings 9a, the upper edges of which are beveled to provide a frustoconical seat 9b for an O-ring 15, which is compressed by a metal or plastic material sleeve 16 fitted on the collector tube generally designated at 10.

The ring 16 may be composed of a metal or plastic material and the sealing force which, as briefly described, derives entirely from the weight of the tube.

The collector tubes 10 are thus suspended from the tube sheet 9 in the form of a bundle of spaced-apart, parallel, self-centering and individually replaceable tubular members within the intermediate portion 23 of the housing. The intermediate portion 23 is of cylindrical configuration and, therefore, generally tubular, while being of a length or height substantially equal to that of the tubes 10. At its upper end, the intermediate section is formed with a flange 24 which is bolted or clamped to the flange 4c previously mentioned. At its lower end, a flange 25 provides the means for securing the intermediate portion 23 to the lower portion 26 of the assembly.

Within this lower portion 26, there is provided a further traverse bar 7 which is conductive and serves as an anchor for the corona-discharge electrode system represented at 8. A pair of diametrically opposite, radially extending ducts 5a and 5a surround the traverse 7' with clearance and support insulator 6b for tensioning the corona-discharge electrodes.

Spanning the bottom of the intermediate portion 23 and the top of the lower portion 26 of the housing is a further tube sheet 11 serving as a condensate-collecting floor and of double-wall configuration so that the upper portion 11a is conically divergent downwardly and has the coping shape of a round roof. The planar lower portion 11b rests upon the inwardly projecting radial pedestals 13.

A multiplicity of vertical tube stubs 12 are anchored in the tube sheet 11 and project upwardly coaxially into respective collector tubes 10. This construction is shown in detail in FIG. 2. Thus the tubes 12 may have an outer diameter d which is at least 5 mm smaller than the inner diameter D of tube 10, the outer surface of tube 12 being formed with three angularly equispaced axially extending spacing rings 17 by which the outer tube is centered on the inner stub 12. At its lower end, each tube 12 is formed with a downwardly and outwardly flaring outlet 18 of substantially conical configuration to facilitate shedding of liquid.

The corona-discharge electrode system consists of electrodes 8', 8" or 8" as desired, or electrodes embodying the features of two or more of them. As can be seen from FIG. 4, the electrodes may be composed of a core 8a of one material and a coating 8b of one of the corrosion-resistant metals mentioned earlier.

To assure alignment of the openings 9a of tube sheet 9 and the corresponding openings in the double-wall tube sheet 11, we bore the holes simultaneously. The double-wall construction of the tube sheet 1 1 increases its stiffness and the accuracy with which the tube stubs are positioned. As is also apparent from FIG. 1, a clearance 110 is provided between the inner wall of housing portion 26 and tube sheet 11 to permit the liquid to drain into the space therebelow. To this end, a circular weir 14a is provided around the gas outlet 14 to collect the liquid while a lateral outlet 14b permits drainage of the liquid face.

FIG. 3 illustrates the fact that each of the collector tubes 10 comprises an inner sleeve 21 of synthetic resin, preferably polyvinyl chloride, and a reinforcing sheet 20 of fiberglass containing polyester-resin bands. The inner surface 22 of the sleeve is artificially roughened, preferably by sandblasting with said at appropriate size to a minimum roughness of 15 microns and a roughness range of, say, 15 30 microns. The original roughness of the surface is generally between 2 and 10 microns.

In general, the moisture contained in the gas to be processed (which is introduced in the direction of arrow A) suffices to precipitate the liquid film onto the inner surfaces of tubes 10. This is especially the case when the apparatus is operated at a temperature below the dew point of the gas. When, however, the apparatus is operated at a temperature above this dew point, e.g., when the dew point of the gas is very low below for example 30C and the relative humidity of the gas is low, nozzles 27 or the like can be used to introduce water or an electrolyte such as dilute sulfuric acid. These nozzles can also be used for a periodic rinsing of the collector tubes while the apparatus is in an inoperative state.

Each of the collector tubes 10 is electrically connected wuth the tube sheet 1 1 by synthetic-resin textile bands 19 or bands composed of a conductive material. The bands 19 may be cemented or welded to the inner faces of the respective tubes 10 and to the upper surface lla of the tube sheet 11. Since the fibers of the synthetic-resin textile bands rapidly become coated or saturated with the liquid, its conductivity resembles that of the inner wall of the tube 10. If it is desired to increase the conductivity, the bands 19 can be composed of graphitized fibers. Synthetic resin and/or graphitized fibers have the important advantage that they are significantly less expensive than corrosionresistant metals.

The electrodes 8 may be provided with smooth surfaces as shown at 8', angularly or axially spaced pointed tips (8") and/or axially and angularly spaced tabs and tongues 8" as previously described.

We claim:

1. An electrostatic precipitator comprising a housing, an array of tubes within said housing, means for feeding gas to said tubes at one end thereof, means for discharging gas at the other end of said tubes, and respective corona-discharge electrodes passing through said tubes, said tubes having artificially roughened internal surfaces with a minimum roughness of 15 microns for maintaining continuous liquid films therein and being composed of synthetic-resin sleeves reinforced externally by synthetic-resin bands containing glass fibers.

2. The electrostatic precipitator defined in claim 1 wherein said housing comprises an upper portion formed with a gas inlet, an intermediate portion connected with said upper portion and enclosing said tubes, and a lower portion connected through said intermediate portion and forming a gas outlet, said electrostatic precipitator further comprising upper and lower tube-sheet members spanning said housing at opposite ends of said intermediate portion and positioning said tubes therein, support members on said housing for said lower tube-sheet member, and traverse members respectively spanning said upper and said lower portion and anchoring opposite ends of said coronadischarge electrodes.

3. The electrostatic precipitator defined in claim 2 wherein at least one of said members is composed of synthetic-resin-coated metal.

4. The electrostatic precipitator defined in claim 2 wherein said lower tube-sheet member is provided with a downwardly divergent upper surface and is formed with an array of tube stubs traversing said lower tube sheet member and respectively coaxially received with clearance within said tubes.

5. The electrostatic precipitator defined in claim 4 wherein each of said stubs is provided with at least three angularly equispaced axially extending ribs for centering each of said tubes on the respective stubs, said stubs having inner diameters at least 5 mm greater than the outer diameters of the respective tube stubs.

6. The electrostatic precipitator defined in claim 4, further comprising weight-operated sealing means suspending each of said tubes on said upper tube-sheet member.

7. The electrostatic precipitator defined in claim 4, further comprising a respective synthetic-resin textile band connecting the interior surface of each of said tubes to the upper surface of said lower tube-sheet without air bubbles or voids therebetween.

l I I '0' k 

2. The electrostatic precipitator defined in claim 1 wherein said housing comprises an upper portion formed with a gas inlet, an intermediate portion connected with said upper portion and enclosing said tubes, and a lower portion connected through said intermediate portion and forming a gas outlet, said electrostatic precipitator further comprising upper and lower tube-sheet members spanning said housing at opposite ends of said intermediate portion and positioning said tubes therein, support members on said housing for said lower tube-sheet member, and traverse members respectively spanning said upper and said lower portion and anchoring opposite ends of said corona-discharge electrodes.
 3. The electrostatic precipitator defined in claim 2 wherein at least one of said members is composed of synthetic-resin-coated metal.
 4. The electrostatic precipitator defined in claim 2 wherein said lower tube-sheet member is provided with a downwardly divergent upper surface and is formed with an array of tube stubs traversing said lower tube-sheet member and respectively coaxially received with clearance within said tubes.
 5. The electrostatic precipitator defined in claim 4 wherein each of said stubs is provided with at least three angularly equispaced axially extending ribs for centering each of said tubes on the respective stubs, said stubs having inner diameters at least 5 mm greater than the outer diameters of the respective tube stubs.
 6. The electrostatic precipitator defined in claim 4, further comprising weight-operated sealing means suspending each of said tubes on said upper tube-sheet member.
 7. The electrostatic precipitator defined in claim 4, further comprising a respective synthetic-resin textile band connecting the interior surface of each of said tubes to the upper surface of said lower tube-sheet member.
 8. The electrostatic precipitator defined in claim 4 wherein each of said corona-discharge electrodes has at least an outer surface composed of a corrosion-resistant metal selected from the group which consists of lead, titanium, tantalum and tungsten.
 9. The electrostatic precipitator defined in claim 4 wherein said corona-discharge electrodes are provided with corona-augmenting projections in angularly and axially spaced relationship along the corona-discharge electrodes.
 10. The electrostatic precipitator defined in claim 4 wherein said sleeves are composed of polyvinyl chloride, said bands are composed of fiberglass, reinforced polyester and said bands are applied to said sleeves without air bubbles or voids therebetween. 